Is the 350lb figure before or after the 90 lb loss?
I can offer no real advice other than to aim for a round figure.
Walk as far as you can each day. Walking is far better exercise than cycling for burning calories. A bike will carry you like a sack. Walking means you must carry the load yourself. Which seems only fair.
Promise yourself a nice safe ordinary bike when you reach 300lbs. But keep walking!
Promise youself a faster road bike when you're 250lbs. But keep walking!
Promise yourself a real lightweight mountain bike when you're 200lbs. Now you're probably running anyway!
In the meantime stop drinking
anything with sugar in it.
Stop eating anything
at all between healthy meals.
Don't have sugar, coke, sweets, biscuits (cookies) chocolate or cakes in the house! Learn to love water. Drink pints of it if you're thirsty! There is no rule that says you must consume commercial sugar-syrup drinks.
My wife and I ask the same thing every day: "Why are fat people always eating or sucking on a bottle of coke like a baby?" Don't believe us? Then open your eyes and look around you. I noticed years ago that my fat colleagues never stop eating or drinking syrup coke. My slim colleagues (and I) eat sandwiches in the lunch hour and drink water or low-fat milk.
A proportion of the population is literally
addicted to
sugar. There is no other explanation for their self-destructive behaviour.
Assuming you really want to ride a mountainbike. Then you have to face the simple, awful, terrifying truth. Even if you exercise every day until you drop from exhaustion. It won't help very much in the long term unless you stop consuming sugary foods and drink.
A very large, young lady literally rode into me, while I was walking on a cycle path, a couple of years ago. It would have been much more fun if she had been 100lbs lighter and I had been thirty years younger!
Best regards
Chrisbee